1. Field of the Invention
The present invention relates to a process and apparatus for coating alkali and alkaline earth metals and more particularly lithium onto a substrate.
2. Description of the Prior Art
Presently, there is a high level of interest in industry in designing thin layer lithium batteries. These batteries include a lithium anode, a transition metal oxide-polymer composite as a cathode, and an electrolyte which may be a solid or a liquid and which includes a dissolved lithium salt.
A principal objective of the designers of these batteries, particularly in applications in which large electrode areas are needed, is to make them as thin as possible while satisfying market needs in terms of capacity, current density, shelf-life and the like.
While methods for making lithium anodes are known, these methods typically provide an anode containing much more lithium than is necessary to meet the electrochemical requirements of the cell. As a consequence, lithium is wasted, the battery is more expensive, and the battery is substantially thicker than necessary. For example, the most common method for fabricating lithium anodes is cold extrusion, but it is difficult to extrude lithium metal into strips thinner than about 100 microns. U.S. Pat. No. 3,721,113, describes a method for alleviating this difficulty by rolling the lithium between smooth polymeric surfaces having sufficiently low critical surface energy to prevent adhesion, however, even this method is limited to thicknesses not less than about 40 microns. In addition, pre-produced lithium strips having a thickness of less than 50 microns are extremely expensive. As such, they do not present a commercially attractive alternative.
Other methods for coating lithium are known in the art as illustrated by U.S. Pat. No. 3,551,184 to Dremann et al. which involves rubbing a heated substrate with a rod of lithium metal and U.S. Pat. No. 3,928,681 and European Published Application No. 285,476 wherein metal substrates are coated as they are conveyed through an alkali metal melt or across a roller which has been immersed in the alkali metal metal. Each of these methods has drawbacks which would make them difficult to implement in an industrial setting. For example, if the apparatus according to European Published Application No. 285,476 would unexpectedly shut down, the roller could quickly corrode and the apparatus would be rendered inoperable.